Electron discharge tube



April 10, 1934.

R. R. MACHLETT 1,954,016

ELECTRON DISCHARGE TUBE Filed Feb. 7, 1953 6 1?) 35x44 EMMZMFQM ATTORNEYS Patented Apr. 10, 1934 UNITED STATES PATENT OFFICE Application February 7,

2 Claims.

This invention relates to X-ray tubes and is concerned more particularly with a novel cathode for use in X-ray tubes which operate at high voltage and in which a broad focal area on the anode is desired.

Tubes of the type described are now widely used in X-ray therapy, operating with voltages ranging from to 300 kilovolts and sometimes more in order to produce the hard X-rays desired for such purposes. In such tubes, the cathodes commonly employed are metal filaments having the form of fiat spirals or small helixes and at the operating voltages mentioned, the cathodes are subjected to great electrostatic stresses. The larger the focalarea the more exposed these filaments must be and since the filaments become soft upon heating in operation and are thus readily affectedby the forces to which they are exposed, special means are required to prevent their distortion by the electrostatic attraction of the anode.

In such broad focus tubes as heretofore constructed, it has been common practice to employ a focusing cup surrounding the cathode to confine the stream of electrons, and as a protective means for the cathode, a. stiff wire is mounted in the opening of the cup to extend toward the anode, this wire taking the electrostatic pull of the anode. In some instances, a fine wire netting stretched over the open end of the cup has been utilized as a protective means instead of the was. In addition to thenecessity of providing protection for the cathode, abroad focal area on the anode is desired in tubes of the type described in order that the anode may better dissipate the large amountsof energy expended in operation and thus have a long life. In the most usual form of tubes heretofore made, the broad focal area on the anode has, been obtained by means of the heavy wire used for protecting the cathode. The effect of this wire mounted in the manner described is to increase the space charge due to the crowding of the electrons around the wire as a result or" the electrostatic field oi the focusing cup. The electrons are caused by the cup to converge around the wire and are then caused by the wire to spread out, thus increasing the focal area and giving it the general form of a ring. If the stiff wire is not employed, the cup is usually increased greatly in size in order to obtain the desired large focal area, but in either case, focusing depends on the amount of the space charge produced by the focusing cup.

In tubes thus constructed, the size of the focal 1933, Serial No. 655,568

area is markedly affected by the applied potential,

and when the focal area is large with respect to the area of the face of the anode, operation at less than usual operating potential may result in a part of the electron stream missing the target entirely, with consequent bombardment of the glass wall to the rear of the target, resulting in serious detriment to the tube. In addition to having this objectionable feature, such tubes are not entirely stable in operation because a relatively small temperature change in the parts of the tube causes a change in the temperature of the closely confined filamentary cathode. This results in a variation in the filament emission and the operating conditions of the tube change accordingly. As a consequence, constant conditions of operation can be obtained in such tubes only by continual regulation of the filament temperature.

The focusing of electrons issuing from a cathode is determined by the configuration of the electrostatic field between the anode and the cathode and this field may be resolved into two components. One of these components is made up of charges on the surfaces of the anode and cath- 1 ode and is thus dependent only on the geometry of the tube and is unailected by changes in filament emission, current, and voltage. The other component is due to the space charge of the electron cloud distributed between the anode and the cathode and varying in shape, size and density with changes in filament emission, current, and voltage. To approach optimum stability, therefore, the first component, which is constant in a given tube, should be as large as possible, while i the second component, which is variable, should be as small as possible so that its effect on focusing is minimized.

The present invention is accordingly directed to the provision of an X-ray tube in which greater stability of operation is obtained than is possible in prior tubes and the disadvantages of the prior tubes are overcome. These results are secured by the use of a cathode of novel construction which, because of its shape, is capable of withstanding electrostatic forces to which it is exposed and is,

in effect, self-protecting. In addition, the new cathode may provide a large focal area on the anode without substantial dependence on the space charge in front of the cathode and its construction is such that heat radiated from other parts of the tube has little eifect on its operating temperature. Thus, in a tube employing the new cathode, the variable component which effects focusing is minimized and the variations in thatcomponent resulting from heat radiation are also decreased.

The new cathode has the form of a loop or series of loops of filament wire with the rounded ends of the loops opposing the anode and it is so mounted that it is well able to withstand the electrostatic forces exerted upon it withoutbeing distorted.

For a better understanding of the invention reference may be had to the accompanying drawing, in which Fig. 1 is a view of an X-ray tube containing one form of the new cathode, parts of the envelope and the cathode assembly being broken away;

Fig. 2 is a longitudinal sectional view through the new cathode assembly, and

Fig. 3 is an end view of the cathode.

Referring now to the drawing, the tube illustrated in Figure 1 comprises the usual glass envelope 10 containing a large area anode 11 constituting the target for the cathode rays and mounted on the usual stem 12, and the new cathode assembly, generally designated 13.

The cathode assembly is mounted on the glass stem 14 by means of rods 15 on which is mounted a metal clamp 16. Leads 17 pass through the stem and clamp and are insulated from the clamp, these leads extending though an opening in a metal disc 18 supported on rods 19 held at one end in the clamp. The leads 17 are connected to the ends of the cathode 20 which consists of a wire filament formed in a series of loops which lie substantially in a cylindrical surface with their rounded ends directed toward the anode. The cathode shown includes three such loops, although a greater or less number may be used as desired, and the rods 15 which pass through the opening in the disc 18 are attached to the cathode between its loops to provide support for it. A tubular metal shield 21 connected electrically to one of the leads 17 is mounted on the glass stem 14 to enclose the assembly and this tube has an open end through which the ends of the loops of the filament project toward the anode.

With the construction illustrated, the electrostatic force exerted on the cathode filament wire is generally in the direction of the axis of the cathode and, since the filament wire is supported at a plurality of spaced points, it is well able to withstand the strain without being broken. Upon the rounded ends of the wire, the negative potential is at its greatest and most of the emission takes place from these ends. With this cathode, the electrons acquire their direction principally from the field between the anode and the cathode rather than by being directed by means, such as .a focusing cup, associated with the cathode. The

electrons consequently strike the anode in a manner which is practically unaffected by the potential applied. In the new tube it has been found that radiation from the anode stem and parts of the anode other than its face is greatly reduced with consequent increase in efficiency.

The tubular shield 21 together with a plate 18 performs the function of preventing positive ions from striking the glass seal at 22, but since the loops of the cathode extend beyond the shield 21, the latter has little or no electrostatic effect upon the electrons which come from the cathode, excepting only those few which come from that portion of the cathode which lies within the shield. Although with the cathode described, the distribution of theimpinging electrons on the anode is not entirelyuniformand is somewhat greater opposite the spaces between the extending loops, the focal area may readily be made broad enough to avoid any detrimental effect on the target surface. Actually the focal area consists of a series of overlapping images of the filament loops and with a cathode of the form illustrated it is roughly triangular.

In the new X-ray tube, the potential effect of focusing and the stability of operation are obtained only when the filament is negative and is operating at a high potential of the order of thousands of volts and the electrical characteristic of the new tube is such that the current is practically independent of the applied voltage in the range of useful values.

I claim:

1. An X-ray tube which comprises an envelope, an anode having a fiat face within the envelope, an electron-emitting cathode within the envelope, said cathode being a filament wire bent to form a succession of loops having their ends extending toward the anode and a shield associated with said cathode and having an opening through which portions of said loops extend, whereby focusing of the electrons'on the anode is mainly accomplished by a constant component of the electrostatic field between the anode and the cathode.

2. An X-ray tube which comprises an envelope, an anode of fiat disc form within the envelope, an electron-emitting cathode within the envelope consisting of a filament wire bent to form a plurality of blunt loops each having a rounded end extending toward the anode, a shield associated with said cathode and having an opening through which a minor portion of each loop extends and is exposed, said exposed portion of the loop lying in the symmetrical electrostatic field between the anode and cathode and said shield being effective to limit the major portion of the electron emission to the exposed portions of the loops and to direct the minor portion of the electron emission from the remainder of the loops toward said anode.

RAYMOND ROBERT MACHLETT. 

